Fiber optic communication systems are becoming prevalent in part because service providers want to deliver high bandwidth communication capabilities (e.g., data and voice) to customers. Fiber optic communication systems employ a network of fiber optic cables to transmit large volumes of data and voice signals over relatively long distances. Optical fiber connectors are an important part of most fiber optic communication systems. Fiber optic connectors allow two optical fibers to be quickly optically connected and disconnected.
A typical fiber optic connector includes a ferrule assembly supported at a front end of a connector housing. The ferrule assembly includes a ferrule and a hub mounted to a rear end of the ferrule. A spring is used to bias the ferrule assembly in a forward direction relative to the connector housing. The ferrule functions to support an end portion of at least one optical fiber (in the case of a multi-fiber ferrule, the ends of multiple fibers are supported). The ferrule has a front end face at which a polished end of the optical fiber is located. When two fiber optic connectors are interconnected, the front end faces of their respective ferrules abut one another and the ferrules are forced together by the spring loads of their respective springs. With the fiber optic connectors connected, their respective optical fibers are coaxially aligned such that the end faces of the optical fibers directly oppose one another. In this way, an optical signal can be transmitted from optical fiber to optical fiber through the aligned end faces of the optical fibers. For many fiber optic connector styles, alignment between two fiber optic connectors is provided through the use of a fiber optic adapter that receives the connectors, aligns the ferrules and mechanically holds the connectors in a connected orientation relative to one another.
Connectors are typically installed on fiber optic cables in the factory through a direct termination process. In a direct termination process, the connector is installed on the fiber optic cable by securing an end portion of an optical fiber of the fiber optic cable within a ferrule of the connector. After the end portion of the optical fiber has been secured within the ferrule, the end face of the ferrule and the end face of the optical fiber are polished and otherwise processed to provide an acceptable optical interface at the end of the optical fiber.
Connectors can also be installed on fiber optic cables using an optical splice. The optical splice can be mechanical splice or a fusion splice. Mechanical splices are often used for field terminated connectors. Fusion splices can be used to fusion splice the optical fiber of the fiber optic cable to the rear end of an optical fiber stub secured within a ferrule. United States Patent Application Publication No. US 2014/0064665 A1 discloses example splice-on connector configurations.
It is desirable for the optical fibers of two mating fiber optic connectors to make physical contact (e.g., glass-to-glass) when an optical connection is made between the mating connectors. With the optical fibers of the fiber optic connectors in physical contact with one another, the optical path behaves as though the glass fiber is continuous. In contrast, an air gap between the optical fibers of the mated connectors will result in an increase in loss due to Fresnel reflections at the air/glass interfaces. In addition, the reflected light will cause a reduction in return loss. This change in return loss is present for angled polished connectors (APC), but is particularly problematic for fiber optic connectors having ferrule end faces that are perpendicular to the optical axes of the optical fibers. These effects can be minimized by applying anti-reflection coatings to the end faces of the optical fibers, but this is often inconvenient. Alternatively, an index matching material such as a gel or oil can be placed between the optical fibers of the mated connectors. This is often undesirable, due to perceived shortcomings of index matching materials, including degradation of the materials over time and/or at high optical power, and the attraction of dust.
Fiber optic connectors have been developed for reducing signal loss at a connector-to-connector interface by expanding the beam diameter of the optical signal that propagates between the mated connectors. U.S. Pat. Nos. 7,031,567; 7,155,096; and PCT International Publication No. WO 2015/013262 disclose fiber optic connectors in which a graded index (GRIN) optical fiber is used as a lens to provide an expanded beam connection at a connector-to-connector interface. Expanded beam connectors also can reduce sensitivity to dust and can be used in high power applications to reduce the intensity of light at the mating connector interface.
The amount of expansion provided by a GRIN lens is highly dependent upon the length of the GRIN lens. Thus, the length of the GRIN lens should be precisely controlled. This can be problematic in situations where a GRIN lens is provided at the end face of a ferrule of a fiber optic connector because polishing of the end face will alter the length of the GRIN lens thereby changing the collimating or focusing properties provided by the GRIN lens. This issue can be overcome by providing an air gap between the GRIN lens and the end face of the ferrule. However, as discussed above, air gaps can cause loss due to Fresnel reflections at the air/glass interfaces. Improvements are needed in this area.